Paper mills have for many years made extensive use, for the cleaning of paper making stock, of screening apparatus embodying a cylindrical perforated screening member defining inlet and accepts chambers on the opposite sides thereof in a closed housing and including a rotor member which operates in one of the chambers to keep the screening perforations open and free from solid material tending to cling to the screening surface. Commonly, the stock or furnish is delivered to the inlet chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collected and discharged from its opposite end.
The assignee of this invention has manufactured and sold many such screens in accordance with a series of U.S. patents, commencing with Staege U.S. Pat. No. 2,347,716, and followed by Martindale U.S. Pat. No. 2,835,173; Seifert U.S. Pat. Nos. 3,849,302 and 4,105,543 and Chupka, Seifert U.S. Pat. No. 4,155,841. Starting with the construction shown in the Martindale patent, all such screens manufactured and sold by applicant's assignee have been characterized by a rotor comprising bars or vanes of airfoil section moving in closely spaced but non-contacting relation with the surface of the screening cylinder for the purpose of creating alternating positive and negative pressure waves effective on the perforations in the screening cylinder to prevent plugging thereof.
The art has experimented widely with detailed variations in screens of the above type, including variations in the vane shape and other forms of rotor, and also in the size, configuration, and spacing of the perforations in the screening cylinder. Thus in the era of the Staege patent in the mid-1940's, the screening cylinder was fabricated from plate material with multiple uniformly cylindrical drilled perforations. When this drilled plate was rolled into a cylinder, a natural result of the rolling operation was to effect some constriction and expansion, respectively, of the inner and outer ends of the drilled holes which were on the inlet (inside) and accepts (outside) surfaces of the resulting screening cylinder. This led in due course to the practice of relieving the accepts end of each cylindrical perforation by a conical bore or countersunk portion to minimize the possibility of plugging.
In more recent years, the trade has been offered pressure screens generally of the above type wherein the perforations in the screening cylinder are elongated slots rather than round holes, typical such constructions being shown in Lamort U.S. Pat. No. 3,617,008; Holz U.S. Pat. No. 3,581,903, and the above noted Seifert '302 and Chupka-Seifert patents. Both Lamort and Holz show slotted screening cylinders wherein the slots have parallel sided portions on the inlet side of the cylinder, but on the accepts side, each slot has widely diverging side walls. Similarly in Chupka-Seifert, the screening cylinder is fabricated from wire of triangular section with the base of the triangle on the inlet side of the cylinder.
This practice of providing relief on the accepts side of the perforations in the screening cylinder of a pressure screen has thus been followed consistently in the paper industry since it was first introduced. The art has demonstrated a conviction that this practice is necessary to minimize the possibility that the perforations would be plugged by fiber and/or contaminant particles unless their minimum dimension is at their inlet ends. Indeed, this conviction has been so firm that all of the claims of the Holz patent include limitations to a "boat-shaped" configuration of the screening slots on the accepts side of the screening cylinder.